Fish are usually dioecious animals. The sex glands are represented by paired testes (milk) in males and ovaries in females.
Eggs – eggs – ripen in the ovaries.
In the testes, male reproductive cells (sperm) mature.
Reproduction of fish. Spawning
The reproduction process in fish is called spawning, and the movement of fish to spawning grounds is called spawning migrations.
When the germ cells mature, the instinct of reproduction is manifested in fish, and they move to places most favorable for the development of their future offspring. There, the females spawn eggs into the water. Males douse the eggs with semen containing sperm. Such fertilization of eggs is called external.
After the fertilization process (the fusion of the sperm with the egg), a multicellular embryo develops in the eggs. After the completion of this process, larvae emerge from the eggs. At first, they live off the remnants of the nutrients of the egg – the yolk sac, and after its disappearance they begin to feed on microscopic algae, ciliates.
Growing up, the larvae switch to feeding on larger animals – daphnia, cyclops or other crustaceans, become similar to adult fish and differ from them only in small size. Young fish are called fry.
The amount of eggs laid in different fish species varies greatly. In most species, each adult female is capable of laying hundreds of thousands of eggs annually. Thus, the female river perch spits up to 300 thousand eggs, and the female moon-fish – up to 300 million.
Such a huge fertility of fish is due to the fact that most of the offspring die for various reasons or become easy prey for predatory animals. Eggs, larvae and fry are eaten by other fish, jellyfish, crustaceans, frogs, larvae of some insects.
Caring for offspring
In fish with low fertility, care for the offspring is manifested. Such fish look for places for laying eggs and protect them from enemies at all stages of development. They lay eggs in the depressions of the bottom (salmon), build nests (stickleback), carry eggs in their mouths (tilapia) or on their bodies (needle fish, seahorses).
The concern for the offspring in fish is unconscious (instinctive). The instinct of “caring” works only as long as the larvae are helpless. When they start to swim well, their parents leave them.
Some species of fish, for example, aquarium guppies, mollienesia, swordtails, are preserved in nature due to live birth. Fertilized eggs linger in the oviducts of females, and the larvae that develop from them are born capable of independent life. Viviparity is also found in sharks.